This invention relates to input conditioning circuits for reading the status of AC or DC inputs, and is especially directed to an AC synchronized input conditioning circuit which could be employed with a microprocessor-controlled thermostatically actuated device, such as a gas-fired condensing furnace.
Typically, modern heating equipment is controlled and actuated by a 24-volt AC thermostat and a number of additional switches for auxiliary equipment, such as a humidifier. The condition of a number of switches and loads must be sensed and fed to the microprocessor via a microprocessor data bus. The thermostat typically also has a rechargeable battery which is charged from the 24-volt AC thermostat power, so the conditioning circuit must be capable of sensing on and off conditions for both AC and DC loads, which may or may not be similarly grounded. The conditioning circuit should also have a high capability for AC line noise rejection, and quick responsiveness, preferably within one or two cycles of the AC power.
Low-pass filters or Resistor-Capacitor-Diode rectifying and integrating circuits have been employed to sense whether a voltage is being applied to a load, or to sense whether a switch is closed or open. Two problems arise from this approach. First, because of the time required to charge their capacitors, these circuits have a rather long response time and would be incompatible with the rather fast electronic furnace control algorithm of the furnace control microprocessor. Second, when a switch is open, a voltage passes through the load 180 degrees out of phase and of a low level. This AC voltage proceeds through the diode to charge the capacitor of the low pass filter circuit. Consequently, the circuit provides a high signal, indicating a positive "on" value even when the associated switch is open.
To avoid the above problems, it has been proposed to employ analog-to-digital converters or voltage dividers, which would have to be externally synchronized with line voltage, and which would rely on a microprocessor. The latter was intended to perform the required synchronization activity. This proposal was rather too costly for the function performed, as the resources could be better employed for system control.